1. Field of the Invention
This invention relates to electrostatic powder spray coating apparatus, and in particular, but not exclusively, to electrode arrangements for electrostatic powder spray coating apparatus.
2. Introduction
Electrostatic powder spray coating apparatus can be used for depositing powder coatings on substrates. A known electrostatic powder spray coating apparatus generally comprises a nozzle through which powder can be sprayed and a discharge electrode located adjacent to the nozzle. The discharge electrode is maintained at an elevated electrical potential with respect to a grounded workpiece so that an electromagnetic field is created between the discharge electrode and the workpiece. As the powder passes through the field, it becomes charged, so that it is attracted to, and adheres to, the workpiece.
Electrostatic spray guns of the corona discharge type operate by the discharge of a very high DC voltage, typically between 30 to 100 kV. This high voltage at the discharge electrode or electrodes results in a discharge current which creates ionized air through which the sprayed powder passes as it is conveyed in an air stream to the earthed product to be coated. As the powder passes through the ionized air some charge is transferred from the ionized air to the powder and this causes the powder to be attracted to anything at a lower potential, for example the earthed product.
There are 3 commonly recognized drawbacks with this method of charging, namely “Faraday cages”, “back ionisation” and “orange peel”.
A Faraday cage is where the charged powder follows the electrostatic lines of force between the discharge electrode and the earthed work piece. These lines of force can be beneficial to create a wrap around effect and coat areas of the product which face away from the spray gun e.g. back surfaces, but they can cause a problem when coating into recesses as the lines of force are established to the outer edges of recesses and will not penetrate inside. This can make it very difficult to coat the inside of such recesses.
Back ionisation is where the excess charge from the ionized air is entrapped into the powder layer deposited onto the surface of the product, and as the charge is all of the same polarity, repulsion effects can cause charge concentrations particularly in thicker coatings, which can erupt from the deposited powder layer to leave holes and craters in the finished and cured powder film.
Orange peel is where the finished cured powder film has an uneven rippled effect like a fine hammer finish. This is believed to be caused largely by excess charge from the ionized air being attracted to the surface of the deposited powder and when the powder melts during the curing process, this excess charge is drawn towards the surface of the substrate causing indentations in the finished powder film.
In summary, the known arrangement has the disadvantage that that the nature, i.e. the strength and distribution, of the field depends largely on the geometry of the workpiece and the proximity of the discharge electrode to the workpiece. Where the workpiece has a complex shape such as sharp edges, undercuts etc., the field tends to concentrate at those regions, which can give rise to preferential coating, and hence, an uneven coating thickness in those areas. The field lines are also established strongly between the discharge electrode and the external edges of recesses thus creating “Faraday Cages” within the recesses which are notoriously difficult to coat as the field is weak or non existent within deep recesses.
In order to charge the coating powder effectively, significantly more charge is generated by the discharge electrode in terms of free ions than is actually required to charge the powder. These free ions are attracted to the earthed workpiece and although most are “neutralised” by the contact with earth, a significant quantity become entrapped in the powder layer or remain on the surface of the powder layer insulated from earth by the powder and can give rise to graters or holes in the finished powder film or an “orange peel” effect on the finished surface.
There are various ways in which these 3 problems can be minimized, for example the operator can make continual adjustment to the discharge voltage and/or discharge current to suit the distance of the gun to the product and the type of product/powder being sprayed. This is not very practical on a busy production line.
Many spray guns/control systems have pre-selected settings of charge for different types of product, e.g. flat sheets; complex parts with Faraday Cages or Recoating where an insulating powder film already exists on the product. Although this can be useful, most products do not fall simply into one category and to change presets while spraying one part or between many different parts is also not very practical.
Many spray guns/control systems now operate with “Constant Current” circuits whereby the discharge current will rise as the spray gun approaches the earthed product but only to a preset value. If the gun is taken closer, the discharge voltage will automatically reduce progressively which reduces the overall charge as the gun approaches the product. Although this helps, it is the current which represents the amount of free ions or ionized air and this does not reduce as the gun distance reduces.
One system which addresses this is described in U.S. Pat. No. 6,274,202 whereby the discharge current as well as the discharge voltage, reduce as the spray gun approaches the product.
One method which has been used with some success is to locate the discharge electrode and an earthed counter electrode within the body or the nozzle of the spray gun. This generates minimal ionized air and contains the electrostatic lines of force within the spray gun. It is, however very difficult to prevent the counter electrode from becoming contaminated with powder and therefore ineffective after a short while.
The internal charging nozzle mentioned above can be turned inside out with an earthed counter electrode fitted outside and behind a conventional corona charging nozzle. This counter electrode can take the form of a single earthed metal rod or pin electrode or can be a series of earthed pins in an annular array either pointing forward towards the nozzle or tangentially out from the nozzle. This is usually an “add on” offered by many spray gun manufacturers and is usually used to achieve a good high quality, smooth finish. In general the charging of the powder is slower and less efficient. The counter electrode is usually mounted approximately 100 mm. behind the discharge electrode in an attempt to keep the discharge voltage at a maximum. This system will cease to be effective if the nozzle (discharge electrode) is taken closer to the product than the distance between the discharge electrode and the counter electrode, say 100 mm, as is often the case when spraying by hand. Another drawback with this method is that the discharge current is usually very high, typically around 100 μA which can cause powder to fuse onto the corona discharge needle due to the hot corona “glow” at the discharge point, this will reduce the charging efficiency and can lead to sparking due to capacitive discharge. Although in many cases the operator will be able to limit the maximum discharge current from the control system, this will invariably not happen as it tends to be the inclination of most operators to turn controls to maximum in an attempt to improve productivity. Another problem with running maximum Discharge voltage and current continuously is that more consideration must be given to the reliability of the highly stressed high voltage components and also the heat generated by the electronic parts of the spray gun.